import hashlib

def calculate_block_hash(header_data):
    """Performs Double-SHA256 hashing on header data."""
    first_pass = hashlib.sha256(header_data.encode('utf-8')).digest()
    second_pass = hashlib.sha256(first_pass).digest()
    return second_pass.hex()

class BlockHeader:
    def __init__(self, height, prev_hash, data):
        self.height = height
        self.prev_hash = prev_hash
        self.data = data
        self.hash = calculate_block_hash(f"{height}{prev_hash}{data}")

def verify_chain(chain):
    print(f"--- Verifying Chain Integrity (Length: {len(chain)}) ---")

    for i in range(1, len(chain)):
        current = chain[i]
        parent = chain[i-1]

        print(f"[*] Checking Block {current.height} -> Parent {parent.height}...")

        # Verify the link
        if current.prev_hash == parent.hash:
            print(f"    [OK] Link valid. Parent hash matches.")
        else:
            print(f"    [!!] ERROR: Broken link at Block {current.height}!")
            print(f"         Expected: {parent.hash[:16]}...")
            print(f"         Found:    {current.prev_hash[:16]}...")
            return False

    print("\n[SUCCESS] Chain is cryptographically secure and immutable.")
    return True

# --- Simulation ---

# 1. Create a valid chain
genesis = BlockHeader(0, "0"*64, "Genesis Data")
block1 = BlockHeader(1, genesis.hash, "Transaction Data 1")
block2 = BlockHeader(2, block1.hash, "Transaction Data 2")

blockchain = [genesis, block1, block2]

# 2. Verify valid chain
verify_chain(blockchain)

# 3. Simulate an "Attack" (Modifying Block 1)
print("\n--- ATTACK: Modifying Block 1 Data ---")
block1.data = "FRAUDULENT_DATA"
# Note: The attacker doesn't update the hash of block1 or the prev_hash of block2
verify_chain(blockchain)